The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebra includes a vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. An intervertebral disc is situated between each vertebral body to cushion and dampen compressive forces experienced by the spinal column. A vertebral canal, called the foramen, containing the spinal cord and nerves is located posterior to the vertebral bodies. In spite of the complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. For example, the kinematics of the spine normally includes flexion, extension, rotation and lateral bending.
There are many types of spinal column disorders including scoliosis (abnormal curvature and twisting of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain as well as diminished range of motion and nerve function. These spinal disorders may also threaten the critical elements of the nervous system housed within the spinal column.
One particular spinal fixation technique includes immobilizing portions of the spine of a patient by using connecting elements such as relatively rigid orthopedic spine rods that run generally parallel to the spine. Another technique utilizes less rigid connecting elements to provide a more dynamic stabilization of the affected regions of the spine. One example of such a spinal stabilization system is the Dynesys® system available from, Zimmer Spine, Inc., of Minneapolis, Minn.
Installation of such systems may be accomplished, for example, by accessing the spine posterially and fastening hooks, bone screws, or other types of vertebral anchors to the pedicles or other bony structures of the appropriate vertebrae. The vertebral anchors may be generally placed in a quantity of two per vertebra, one on either side of the spinal cord, and serve as anchor points for the connecting elements.
It may be desirable for some spinal stabilization systems to have regions of more rigid stabilization and regions of more flexible stabilization. Accordingly, there is an ongoing need to provide alternative apparatus, devices, assemblies, systems and/or methods that can function to alleviate pain or discomfort, provide stability, such as dynamic stability, and/or restore a range of motion to a spinal segment of a spinal column.